Control of a four stroke engine requires that a spark be applied during a compression stroke. Since the engine turns two revolutions, and thus reaches top dead center (TDC) twice each engine cycle, it is necessary to distinguish which occurrence of TDC follows the compression stroke and which occurrence follows the exhaust stroke.
Conventionally, an engine is equipped with two sensors, one rotating with the crankshaft to determine accurate crankshaft position, and one rotating with the camshaft to provide a cylinder identification pulse once every two crankshaft revolutions. Position timing information for each cylinder is derived from the synchronization of these two sensors. It would be preferable, however to use only one sensor. The use of two sensors is more expensive than one as well as consuming more space. The use of two sensors also lowers overall system reliability simply because there are more components for potential failure. In some applications it is desirable to minimize both the cost and the size of the engine, and of course improving reliability usually a desired goal.